There are many ways of heating air used as space heat for domestic and commercial buildings. One way is to employ an air handler in conjunction with a water heater, wherein the water heater supplies hot water to the air handler to generate heated air. Oftentimes, however, the water heater serves the additional function of supplying potable water. Thus, there are instances when the ordinary domestic use of water, oftentimes referred to as “water draw,” are above or equal to the output flow capacity of the water heater. This can lead to conditions where the air handler is deprived of sufficiently hot water flow. Such a loss of water flow to the air handler pump can lead to cavitation of the impeller, thereby considerably shortening the life of the pump. Also, low or no water flow to the air handler can lead to reduced energy transfer through the air handler heat exchanger and lower the delivered air temperature such that the air handler blows cold air into the space instead of the desired heated air.